Forensic officers examine the Vauxhall Vectra left in the car park of the Forth Road mosque in Glasgow. One of the men arrested in Scotland told police about the car. Photograph: Jeff J Mitchell/Getty Images

The plot to mount car bomb attacks in Britain was hatched outside the UK, with the doctors allegedly involved linked to a ringleader or mastermind abroad, counter-terrorism officials believe. One theory is that the alleged plot was orchestrated by one or two jihadists who infiltrated the NHS and indoctrinated others.

It emerged last night that investigators suspect that the two men caught at Glasgow airport trying to ram a Jeep into the terminal building were also behind the failed attempt to detonate two car bombs in central London last Friday.

Sources also suggested that all known members of the cell had been accounted for. "There is not a huge manhunt," one well-placed official said. Though the terrorist threat level remains at "critical" there were indications that it would soon be downgraded to "severe", meaning an attack is highly likely but not imminent.

All eight people arrested have links with the NHS - seven are doctors or medical students and one worked as a laboratory technician. All entered the UK legally.

Intelligence sources last night declined to say where the "guiding hand" or mastermind behind the plot was based. It is likely, given the dates on which some of the suspects entered Britain, that the plot was hatched a year ago, or even earlier.

Though MI5 insists none of the suspects arrested in connection with the plot were under surveillance, the mobile phones detectives recovered from the would-be car bombs contained details that matched material on the security service database. Counter-terrorism officials say data from the phones and email traffic was checked on the database used by MI5, MI6 and GCHQ, the government's eavesdropping centre. Connections were found linking that information and communications abroad, which enabled the police and security services to speed up their investigations in Britain.

"This linkage allowed the police to move quickly," said a source. The foreign intercepts included talk of jihad, an official added. Counter-terrorism officials say the links between members of the British-based cell were via the foreign intercepts. It is believed, for example, that Mohamed Haneef, the doctor arrested at Brisbane airport, had long conversations with one of the suspects arrested in Britain.

Security and intelligence sources said yesterday that no link had yet been made between the failed bomb attacks and al-Qaida. However, they say al-Qaida's tactics are more flexible than they were at the time of the 9/11 attacks and that their sympathisers are increasingly left to decide for themselves the means of attack.

Anti-terrorist officials suggested yesterday that the use of car bombs was a response to warnings to farmers and suppliers fertiliser - the favoured ingredient of previous terror plots - to report suspicious demands for large quantities of the product. The bombers turned to the more easily obtainable fuel and propane gas. The fact that the attacks were bungled suggests the perpetrators did not have the same expertise as British bombers, many of whom were trained in Pakistan.

Separate sources say the attack on Glasgow's airport terminal was intended as a suicide car bombing. It was the first time such an attack had been attempted in Britain. One source said the attack happened as the terrorists felt the police were closing in on them: "They felt the police were on to them; it was a bit of a panic attack."

It emerged yesterday that all three suspects arrested in Scotland after the Glasgow attack, including the alleged passenger of the Jeep Cherokee used in the attack, a hospital doctor called Bilal Abdulla, were secretly transferred to Paddington Green high security police station in London on Monday night.

The driver of the car, Lebanese doctor Khalid Ahmed, is in a critical condition at the Royal Alexandra hospital in Paisley with severe burns. The decision to transfer the other suspects, two men thought to be Saudi trainee doctors working at the hospital in Paisley, was taken on Monday afternoon by the head of the Scottish prosecution service, Eilish Angiolini, QC.

One of the men arrested in Scotland told police he had left a hired car in the car park at Forth Road mosque. Early yesterday morning police bomb disposal experts carried out a series of controlled explosions on the silver Vauxhall Vectra. It was then taken away for forensic examination.

In a separate development, two Asian men were being questioned by detectives in Lancashire last night after they were arrested on a Blackburn industrial estate following a reported delivery of around 10 large gas canisters.

Passengers at Heathrow were delayed for up to five hours yesterday after an alert caused by a suspect package, which proved to be a false alarm.